1. Field of the Invention
The present invention relates to the art of earth boring with rolling cutter drill bits. In particular, this invention relates to an improved friction bearing for sealed and lubricated three cone earth boring bits utilized for gas and oil well drilling.
2. Description of Related Art
Sealed and lubricated rolling cutter drill bits (also called rock bits) typically have three different bearing structures in each cutter. The first bearing structure is designed to support cantilevered radial loads and is typically a journal bearing or a roller bearing. The second bearing structure is designed to retain the rolling cutter upon the cantilevered bearing spindle when the cutter is subjected to offward thrust. This retention system is generally comprised of either ball bearings or a friction bearing such as a snap ring or a threaded retaining ring, and it is this bearing structure to which the present invention relates. The third bearing structure is designed to carry onward axial thrust loads and is most often a friction type bearing.
Analysis of used rolling cutter drilling bits shows that when high loads are combined with high rpm, the thrust bearing often fails or the resulting heat build-up causes degradation of the other bearings. Even though a great many designs and materials for rock bit thrust bearings have been used in an attempt to solve this problem, thrust bearing damage still remains a source of bearing failure, especially at very high rpm.
Friction thrust bearing elements in drill bits can be categorized as either fixed or floating bearing elements. Fixed elements are most often welded inlays or interference fitted inserts in the rolling cutter or in the end of the cantilevered bearing spindle. The most commonly used inlay is a STELLITE (registered trademark) material welded on the thrust surface of the cantilevered bearing spindle. Another inlay features alternating surface areas of load bearing and anti-galling materials on the thrust surfaces as shown in U.S. Pat. Nos. 3,235,316 and 4,323,284. Alternately, a tungsten carbide button can be pressed into the cutter as shown in U.S. Pat. No. 3,384,426 or an irregularly shaped disk can be captured in the cutter as shown in U.S. Pat. No. 4,413,918. These materials must be fixed into the rolling cutter or the cantilevered bearing spindle because they lack either strength or ductility or both. For instance, the welded STELLITE and tungsten carbide inserts have less than 1% ductility. If they were not supported by the spindle substrate they would soon disintegrate in service. The ductile inlays have such low hardnesses and yield strengths they would soon. wear and/or deform excessively in service.
Drill bit performance is increasingly limited by thrust bearing capability as modern drilling applications demand ever higher loads and speeds. Degradation of thrust bearing surfaces via wear, spalling deformation, and fracture is primarily due to overheating. Significant decreases in both tribological and design strength properties with temperatures are well known to directly cause current materials systems failures.
Floating intermediate thrust bearing elements for drill bits are shown in U.S. Pat. Nos. 3,720,274; 4,410,284; 4,439,050 and 5,161,898. These floating thrust bearing elements have advantages over fixed elements well known and practiced in the prior art.
As shown in U.S. Pat. Nos. 4,439,050 and 5,161,898 and implied in several of the other Patents cited above, floating bearing assemblies can be designed as one or more fixed bearing elements mounted or fused on carriers. The entire assembly functions as a single floating bearing element. These designs are not only more expensive to manufacture and occupy more bearing space than unitary material floating bearing elements in drill bits, they are also not as effective under extreme loads. This is because the bearing material can become separated from the carrier and contaminate the other bearing systems.
The magnitude of temperatures reached in the floating thrust elements of drill bits has been generally under-stated in the prior art. Floating thrust bearing elements in drill bits do not transfer heat into the external environment as easily as the cantilevered bearing spindle or the rolling cutter. Consequently, during a peak load event, the surface temperature of the thrust element will increase preferentially over its mating surfaces within the drill bit. After the load event, the heat will transfer relatively slowly from the thrust element through the mating surfaces and into the surrounding environment. The peak operating surface temperatures at the asperities of the floating thrust element can become extremely high, and in fact can exceed 1500 degree F., leading to failure.
To function successfully in a drill bit, a unitary floating bearing element must have acceptable yield strength and ductility at operating temperature. In U.S. Pat. Nos. 3,721,307, 4,641,976 and 5,161,898, the minimum yield strength for a successful floating bearing element material in a drill bit is established at 140,000 psi and the minimum ductility is established to be about 1% to 4%. The Beryllium Copper of U.S. Pat. No. 3,721,307 is still in common use in drill bits, and is specified in ASTM B643-92, temper TH04 (HT). The minimum yield strength of this material is indicated in that specification to be from 140,000 psi to 155,000 psi. The copper based spinodal material of U.S. Pat. No. 4,641,976 with an ultimate tensile strength of 180,000 psi typically has minimum yield strength of from 145,000 psi to 160,000 psi. Quoting U.S. Pat. No. 5,161,898 (Col. 1, line 47),xe2x80x9c. . . materials having minimum yield strengths of about 140,000 psi are needed to avoid macroscopic plastic deformation in service.xe2x80x9d It is understood that all these yield strength values are at room temperature. It is apparent in the prior art that 140,000 psi is common as the minimum room temperature yield strength for floating bearing elements in drill bits.
The copper based alloys of U.S. Pat. Nos. 3,721,307 and 4,641,976 mentioned above have a weakness; however, when the operating temperatures reach about 600 degree F. their strength greatly decreases. Generally, yield strengths of these copper alloys approach zero psi at 1500 degree F.
Historically, failure analysis of drill bit bearings has led bit designers to develop bearing materials with ever higher yield strengths. However, these materials probably performed better not because their room temperature yield strengths were higher, but because raising the room temperature yield strength tended to increase the operating temperature yield strengths. This success, coupled with the ease of measurement at room temperature, has led to a search for materials having high yield strength and good ductility at room temperature. Little attention has been given to materials with lower room temperature yield strength, but relatively high yield strength at operating temperature.
The friction bearing typically takes the form of a beryllium copper split tube surrounding part of the spindle, the bearing being located within a chamber, defined between the spindle and the rolling cutter, appropriate seal arrangements being provided to restrict the ingress of well fluid into the chamber and the egress of lubricant from the chamber. There is a tendency for the rolling cutter to reciprocate, axially, on the spindle, in use. As a result, the fluid pressure within the chamber varies significantly. Attempts have been made to stabilize the chamber pressure, for example by connecting the chamber to a reservoir divided by a flexible diaphragm which is movable to vary the volume of the part of the reservoir connected to the chamber, thereby absorbing pressure fluctuations. The movement of the rolling cutter is sufficiently fast that such a technique does not adequately stabilize the fluid pressure. As a result, leakage of fluid past the seals occurs. The ingress of fluids may result in particulate matter entering the chamber and this may result in increased levels of wear.
The present invention is directed at a new friction bearing element for rolling cutter rock drill bits formed of a material selected from the class of materials formed primarily of cobalt and chromium carbide and known as wrought, ductile STELLITES. One material of this class is Aerospace Material Specification AMS 5894, a cobalt based, wrought super alloy commercially available by the trade name STELLITE, grade 6B, from Del Oro Stellite, Ontario, Canada. Friction bearings made from this STELLITE material with a room temperature yield strength averaging 105,000 psi have been successfully field tested in multiple drilling environments.
The use of a friction bearing constructed from such a material is advantageous in that the bearing can be of reduced thickness compared to typical bearings. The reduction in the thickness of the bearing permits the chamber between the spindle and the rolling cutter to be of reduced dimensions. As described hereinbefore, the rolling cutter tends to reciprocate, acting like a piston, but by reducing the dimensions of the chamber the xe2x80x9cswept areaxe2x80x9d of the piston-like motion of the rolling cutter can be reduced and this, in turn, can reduce the magnitude of the pressure fluctuations and the risk of leakage past the seals.
The invention provides a rolling cutter drill bit with a friction bearing element of tubular form made of ductile STELLITE having a room temperature tensile elongation greater than 4% and a room temperature yield strength less than 120,000 psi.
The invention also provides a rolling cutter drill bit where the friction bearing element is of tubular form and has a wall thickness of less than 0.05 inch. The wall thickness is preferably greater than about 0.015 inch. Preferably the wall thickness falls within the range 0.02 inch to 0.04 inch, the wall thickness conveniently being about 0.03 inch.
The friction bearing element is preferably provided with a coating of a relatively soft material. The relatively soft material may comprise silver.
The invention further provides a rolling cutter drill bit with a tubular friction bearing element made of ductile STELLITE with a room temperature yield strength less than 120,000 psi, said material further having a yield strength greater than 30,000 psi, an un-notched longitudinal Charpy impact toughness of greater than 120 ft-lbs., and a tensile elongation greater than 10%, all at 1500 degrees F.
The invention still further provides a rolling cutter drill bit with a tubular friction bearing element made of a ductile, wrought STELLITE material.
The invention still further provides a rolling cutter drill bit with a tubular friction bearing element made of a ductile, wrought STELLITE with a room temperature yield strength less than 120,000 psi, and coated with a soft, metallic solid lubricant.